1. Field of the Invention
The invention relates to distillation apparatus and also to separatory distillation processes. In particular, the invention relates to processes to stabilize distillation for inhibiting scale and preventing corrosion, under pressure or vacuum, and also relates to processes for flash vaporization of the distilland. In particular, a method for improved heat transfer is disclosed that is generally applicable to a wide range of fluid treatment applications, and specifically applicable to desalination plants.
2. Description of the Prior Art
Desalination of saline waters has been accomplished by the multi-stage flash (MSF) process, or other processes requiring preheating of the brine streams, such as the vertical tube evaporation (VTE) process, or the combined vertical tube evaporator/multi-stage flash (VTE/MSF) processes. During the heating, they are maintained under pressure so as to prevent boiling or vaporization within the tubes, and the salt waters are heated as a single phase liquid. After attaining the maximum temperature, the brines are allowed to pass successively through a number of flash chambers arranged in series. The pressure in each chamber is lower than in the preceding chamber, and is lower than the temperature at which evaporation of water can occur. Thus, as the brine enters a flash chamber, part of it is vaporized while at the same time the remaining brine is cooled somewhat. The cooled brine then passes into the next, lower pressure chamber where the flash evaporation is repeated. The steam evaporated in each chamber flows into the shell side of the brine heat exchanger, wherein it is condensed as it passes over the outside of the aforementioned tubes or conduits, within which the saline feed water is being heated.
The highest temperature to which the brine is heated within the tubes is normally limited by the potential for formation of scale on the inside of the tubes, such as a calcium carbonate (CaCO.sub.3) or a calcium sulphate (CaSO.sub.4) scale, and by the method used to prevent or inhibit the formation of such scale. The acid-treated plant, wherein the brine feed is pretreated by acidification to cause the conversion of carbonates to carbon dioxide (CO.sub.2) gas, can sustain brine temperatures to as high as 250.degree. F. The chemical treatment plant, where a chemical (often a proprietary mixture) is added to the feed to inhibit scale formation, is often limited to a maximum brine temperature of 180.degree. F. to 200.degree. F.
The design of MSF distillation plants, or the brine heaters of other distillation plants, is constrained by various requirements. For example, the velocity of the saline water must not be less than some minimum value (often 5 or 6 feet per second), so as to prevent fouling of the tube surface. Also, the tube diameter must not be less than some minimum value (often 1/2 to 5/8 inches) to prevent plugging of the tube entrances by blockage due to marine life that could be nourished and grow within the system. These constraints, together with the heat transfer coefficients that are attainable, often lead to a very long plant geometry. For example, a low temperature (190.degree. F. maximum temperature of the saline feed waters) chemically-treated plant may be four hundred feet long; said plant may consist of up to forty flash chambers, each of them being approximately ten feet in length.
Various methods have been proposed to increase the heat transfer coefficient in the heat exchangers of the MSF distillation process and in the brine heaters of other distillation processes such as the VTE or VTE/MSF processes. One method is to change the geometry of the tube surface, so that there are various protrusions or projections, such as with the finned tube, the fluted tube, the corrugated tube, or the spiral tube. Another method is to add a chemical to the steam phase condensing on the outside of the tube, or to change the structure of the outside surface of the tube, so as to promote condensation of the steam as droplets instead of as a continuous film of condensate. These methods are all effective in varying degrees, but are not as effective as the present invention with respect to providing the design flexibility to greatly reduce the size of the desalination plant. In addition, any of these methods could be used together with the present invention, if economically warranted, to provide potentially greater benefits than one method taken alone.
It has been proposed in the past that carbon dioxide (CO.sub.2) gas be dissolved in the saline water under pressure before the saline water is passed through the tubes of the brine heat exchanger. Since CO.sub.2 in water forms an acid, this process, in effect, is simply another form of the acidification pretreatment process previously described for scale prevention; the CO.sub.2 acid is merely substituted for some other acid, such as sulfuric acid. However, this method calls for the essential dissolution of the CO.sub.2 in the liquid saline water, so that the fluid passing through the brine heater is a single phase, all liquid saline stream. In the present invention, if CO.sub.2 gas is used as the gas phase to promote two-phase flow within the brine heater tubes, it would have the incidental effect of acidifying the brine stream, by partial dissolution of the CO.sub.2 into the liquid brine stream, in the same manner as would any other dissolved acid, and would thereby diminish the potential for formation of scale within the tubes.
Also known in the prior art are falling film evaporators, wherein the rate of heat transfer is increased by cycling some of the water vapor through the evaporator tubes. Such an arrangement employs vertical tubes having a thin film of water passing along the inner wall of each tube, and the vapor is recycled and also passed downwardly through the tube to increase the total gas flow rate in the tube, thereby increasing the turbulence level within the tube, which in turn increases the heat transfer rate. This type of evaporator is known only in situations where the gas employed in creating the two phase flow is the vapor of the liquid, and one goal of the process is the production of the vapor. Therefore, such falling film evaporators are not functional in applications where, heretofor, formation of vapor within the evaporator was not a desired end of the process.